In recent years, a high-sensitivity optical communication technique that uses an optical fiber amplifier doped with a rare-earth element to directly amplify an optical signal has been adopted in a long-distance and large capacity optical communication system. In such a system, a transmission optical signal is excited by excitation lights having different wavelengths and caused induced emission to thereby obtain an amplified transmission optical signal. It is known that energy accumulated in the optical fiber amplifier is rapidly emitted in an inductive manner to generate a high-intensity light pulse (surge light) when a signal light is suddenly inputted from the optical fiber amplifier or outputted to the optical fiber amplifier in the optical amplification process. This phenomenon occurs at the signal switching time in a multiple-wavelength transmission or the like, unintended instantaneous interruption of a light signal, or the like. When such a high-intensity light pulse (surge light) is transmitted, optical components mounted in a next stage light apparatus may be degraded or destroyed.
In a recent broadband environment represented by a Fiber To The Home (FTTH), where a light signal is distributed to individual homes, an unintended high power input light may be transmitted from the transmission side (station side) in some cases. This phenomenon easily occurs at, for example, system maintenance time. Additionally, there is a possibility that the high power input light is reversely transmitted from the reception side (customer side) to destroy optical components on the transmission side intentionally. In such a background, a countermeasure for protecting a light device from a surge light or high power input light becomes important.
A fiber-type optical fuse and the like have been proposed as an apparatus that performs operation for protecting an optical apparatus from such a surge light or high power input light. The fiber-type optical fuse uses fiber fuse phenomenon as its fundamental principle. For example, a light-absorbing material or the like is applied to a part of a waveguide area or the diameter of a fiber core is intentionally made to change in a discontinuous manner. Consequently, heat is generated by local absorption or dissipation to thereby allow the fiber core area to undergo fusion or evaporation. With this configuration, a propagating light can be scattered or cut off.
FIG. 11 is a configuration example of a fiber-type optical fuse disclosed in U.S. Patent Application Publication No. 2002-0141021 A1. An input light 115 passing through an optical fiber 111 is amplified through an excitation light source 113 and a fiber amplifier 112. A tapered-type fiber 114 is connected to the output side of the optical fiber 111. A guiding light is dispersed due to a difference in core shape in the tapered-type fiber 114, so that the tapered-type fiber 114 is weak to a high power output light.
With this feature, thermal destruction occurs in the tapered-type fiber 114 area in association with generation of a surge light or the like, so that it is possible to reduce the power of an output light 116.
FIG. 12 is a configuration example of a fiber-type optical fuse disclosed in JPA-11-281842. In this example, a medium 123 comprising a heat-sensitive and heat-metamorphic material and a light-pyrogenic material is provided between optical fibers 121 in a chassis 122. The heat-sensitive and heat-metamorphic material layer has optical transparency and generates heat in accordance with the intensity of an input light 124. Thermal destruction occurs when the optical power exceeds a predetermined level to scatter or cut off a propagating light, thereby reducing a power of an output light 125. Optical fuse operation is thus realized.
FIG. 13 is a configuration example of a fiber-type optical fuse disclosed in JPA-11-274547. The operation principle of this example is the same as that used in the example disclosed in JPA-11-281842. That is, a film body 132 whose transmittance and reflectance is irreversibly changed depending on the optical power of an input light 135 passing through an optical fiber 131 is provided in the middle of an optical transmission path. Heat deterioration due to generation of heat occurs when the optical power exceeds a predetermined level to reduce the transmittance, thereby reducing the power of an output light 136. Optical fuse operation is thus realized.
In this example, a monitor device 133 and a power source and determination circuit 134 quantitatively monitor the degree of the deterioration of the film body 132 by monitoring the intensity of a reflection light.